1. Field of the Invention
The present invention relates to a method for producing an optical part.
2. Description of the Related Art
In various types of optical systems such as imaging systems of cameras and optical fibers and pickup optical systems of copiers and compact disks, a plurality of lenses are necessary for correcting aberrations such as a spherical aberration and a chromatic aberration. In particular, in an image pickup system and an optical system used under a white light source, the number of lenses necessary for correcting a chromatic aberration is larger than that of a monochromatic optical system.
A radial type refractive index distribution lens having a refractive index gradient (hereinafter referred to as GI: Gradient Index) in the radial direction from an optical axis in a lens medium, when it is used as an optical part, shows excellent effect on correction of, in particular, a chromatic aberration. Therefore, the number of the lenses for correcting the chromatic aberration can be decreased, and a reduction in size of a zoom lens and increases in functions, such as widening of angle and heightening of magnification, can be realized.
As methods for generating a gradient of composition ratios or a gradient of refractive indices in a plastic lens, methods employing ion exchange, electric field application, centrifugation, polymerization control, or swelling control are known. Here, in order to increase the GI, fine particles having a high refractive index are dispersed with a distribution in a plastic resin.
U.S. Patent Application Publication No. 2005/0151286 discloses a method for producing a plastic rod lens provided with a distribution of fine particles by utilizing centrifugation.
Furthermore, though it is not a technology for producing optical part but a technology for hologram recording, a method for generating a GI by utilizing radiation curing has been proposed. In Japanese Patent Laid-Open No. 2007-206704, a concentration distribution of inorganic fine particles is generated by irradiating with light an organic component of a matrix that uniformly contains inorganic fine particles and is provided with an interference pattern of bright and dark areas and transferring the fine particles based on a difference in the degree of polymerization of the matrix.
In a radial type refractive index distribution lens, the refractive index distribution in an order of mm is generated. Such a need is not limited to the refractive index distribution and is also applied to a lens having a distribution at a common composition ratio.
However, the GI lens in U.S. Patent Application Publication No. 2005/0151286 is a rod lens, and it is difficult to realize a GI or a composition ratio distribution in an order of mm, which is required in the radial type refractive index distribution lens. Furthermore, in the case of employing centrifugation, the concentration of fine particles is decreased toward the central portion. That is, since only a distribution in which the refractive index increases from the central portion toward the peripheral portion can be obtained, the degree of freedom in design is low.
In Japanese Patent Laid-Open No. 2007-206704, the transfer of fine particles based on a difference in the degree of polymerization is in an order of μm, and therefore the range of the resulting GI is about several micrometers.